1. I shoot a lot of surplus 7.62 x 54r out of my Mosin M44... I don't notice too much of a difference between those and the 7.62 x 54r shells that I buy brand new (other than being a lot dirtier). Those surplus rounds are at least 30 years old, and very probably older than that.
2. Whenever there is a machine or human process of mass production, there absolutely is the chance for defectives. Not sure if this is the case (and you'll probably never know), but it is a possibility.
3. If anything, from reading your post, perhaps it struck a twig or other object that caused it to deviate slightly. That 'very light brush' you mentioned could have easily put the bullet off course by a foot or more if your bullet struck it... no matter how minute it seems when attempting to shoot through it.